The present invention relates to service arrangements for television receivers and, more particularly, to a service circuit for facilitating the adjustment of the cut-off points of the cathode elements of a color cathode ray tube.
Color television receivers normally employ a multi-gun color cathode ray tube with each one of the electron guns providing an electron beam activating the phosphors of a screen in a well known manner for reproducing a transmitted color image. The electron guns, the electron beams and the phosphor areas all vary with respect to one another in each manufactured cathode ray tube necessitating what is usually referred to as "setting-up" or adjusting the color cathode ray tube to achieve optimum operation. In particular, these adjustments typically involve a procedure whereby the bias potential applied to one or more of the cathode ray tube grids is set such that at least one of the electron gun cathode elements will cut-off at a predetermined level of video signal voltage.
In color television receivers using a high level matrixing system i.e. a receiver where the color difference signals are combined with the luminance signal in a high level output video amplifier stage to provide the red, blue and green color signals for driving the electron gun cathodes, the adjustment or "set-up" procedure is typically accomplished by coupling a predetermined DC control voltage or bias potential from the output of the video output amplifier to the cathode of each electron gun and then adjusting a potentiometer coupled to a cathode ray tube grid until at least one of the guns just begins producing an image on the screen. As a consequence, the cut-off point of the electron gun cathode has thereby been set to a video signal level corresponding to the predetermined DC control voltage. Potentiometers in the video amplifier output stage may subsequently be adjusted so that the other electron gun cathodes will also cut-off at the same predetermined DC control voltage. Various circuit arrangements have been proposed in the past for facilitating this adjustment procedure, most of these circuits having one feature in common. In particular, since the luminance signal component of the composite video signal is subject to unpredictable level variations due to the viewer operable brightness control, the luminance signal channel must be disabled to allow the development of the necessary predetermined DC control voltage at the output of the video output amplifier stage. Since the color difference signals, however, vary about relatively constant levels, they may be used to drive the output video amplifier for developing the predetermined DC control voltage.
In the prior art, the foregoing is typically accomplished by means of a so-called service switch which has a normal position wherein the luminance signal is coupled to the video output amplifier for enabling normal operation of the receiver and a service position in which the luminance channel is disabled and a fixed impedance is coupled instead to the video output amplifier, the fixed impedance in association with the relatively constant level color difference signals driving the amplifier for producing the predetermined DC control voltage. U.S. Pat. Nos. 3,670,095 to Arumugham et al and 3,820,155 to Neal are exemplary of such circuit arrangements. U.S. Pat. No. 3,612,757 teaches a similar circuit except that the matrixing of the color difference signals with the luminance signal is done in the cathode ray tube itself instead of in the video output amplifier. Other service switch arrangements used in high level matrixing systems are disclosed in U.S. Pat. Nos. 4,118,729 to Shanley II, 4,123,776 to Cochran et al and 4,130,829 to Kam et al.
The prior art service switch arrangements will not, however, normally be appropriate for use in a television receiver employing a low level matrixing system, i.e. a receiver where the color difference signals are combined with the luminance signal in a low level stage to provide the red, blue and green color signals which are in turn coupled through a high level video output amplifier for driving the electron gun cathodes. In such systems, all of the color signals coupled to the video output amplifier include components directly affected by the viewer brightness control so that their levels are not predictable. It is, accordingly, a primary object of the present invention to provide a service arrangement for enabling adjustment of the cut-off points of the electron gun cathode elements in a color television receiver using a low level matrixing system. It is a further object of the invention to provide an arrangement of the foregoing type which may be inexpensively manufactured and which uses a minimum number of electrical components.